In the recent years, the Community Antenna TeleVision (CATV) network has evolved from an unidirectional analog system to a bi-directional, Hybrid Fiber Coaxial with a mix of analog and digital signals. The most common configuration comprises a fiber optic main distribution network associated with a local distribution network using coaxial cable. The downstream signals are a mix of analog modulated television and Frequency Modulation (FM) plus a few service signals in a band from 50 to 550 MHz, above 550 MHz digital modulated signals in the QAM-64 or QAM-256 are available, plus possibly a few auxiliary digital service signals around 50 to 200 MHz. All of these signals are normally continuous.
In the reverse or upstream path signal from 5 to 42 MHz, mostly digital signals in the Quadrature Phase Shift Keying (QPSK—QAM-4) or QAM-16 formats are used plus a few service signals in either analog or digital modulation. All of these signals are normally pulsed ON when required.
The multiplicity of digital signals induces a requirement to measure the quality of these signals. There have been two trends in performance measurement.
On the one hand, there is a multiplicity of test instruments with somewhat limited capability, i.e. measurement of total impairments (in the form of Modulation Error Ratio (MER) or its inverse Error Vector Magnitude (EVM)), error data rates in a few formats such as Bit Error Rate (BER) before and after standard Forward Error Correction (FEC), data packets loss, etc. Display of received signal in constellation format and Equalizer coefficients complemented these quantified measurements for analysis by the instrument operator.
On the other hand, there are a few instruments for laboratory use that are far more sophisticated but also much more of general purposes. The general configuration of these instruments is a raw signal recording followed by complete demodulation and analysis in software. The analysis will provide the same measurements in terms of MER/EVM but no BER within the format of digital television modulation, but BER Test using pseudo-random sequences. Measurements also include peak constellation deviation, average deviation, eye pattern, EVM variation in time, etc.
Being of general purposes laboratory instruments, these are large, not in-field intended, require elaborate set-up and very qualified personnel to interpret results. Also the cost of these instruments is many times that of the first category.
The operator of a transmission system uses an array of tools to diagnose the state of this channel. Each tool is focused on a specific aspect of the operation. Amongst that variety, we notice the constellation display of the received soft decisions. This is a visual display of the received signal in a quasi-analog fashion. The clusters of received symbols around ideal values provide an indication of impairments by their shape, their spread and their positions.
It takes a trained “eye” to diagnose the various impairments but it is nearly impossible to quantify them, also the occurrence of more that one impairment renders the analysis very difficult. There is then a need to analyze the constellation to extract and quantify impairments.